{ INFORMATICS.MCCME.RU
  Problem statement: http://informatics.mccme.ru/moodle/mod/statements/view3.php?chapterid=1466
  Verdict: Accepted
}
uses SysUtils, Math;
 
var
	n,ans,m,kk:int64;
 
begin
reset(input,'input.txt');
rewrite(output,'output.txt');
read(n);
ans:=0;
if n>0 then begin
	if odd(n) then begin
		kk:=(n+1) div 2;
		ans:=n*kk;
	end else begin
		kk:=n div 2;
		ans:=(n+1)*kk;
	end;	
end else begin
	m:=abs(n);
	if odd(m) then begin
		kk:=(m+1) div 2;
		ans:=1-m*kk;
	end else begin
		kk:=m div 2;
		ans:=1-(m+1)*kk;
	end;	
end;
write(ans);
end.